LATERAL TERMINATION OF STRIKE-SLIP FAULTS The right-lateral San Gregorio-Sur–San Simeon–Hosgri fault system splits from the San Andreas fault near San Francisco, and terminates against the western Transverse Ranges of southern California, 400 km to the south- east. The Hosgri fault is the southern part of this coastal fault system (Fig. 1A). Worldwide, continental strike-slip faults terminate abruptly into thrust faults and blocks that rotate about vertical axes in areas as diverse as Mon- golia (Bayasgalan et al., 1999), the Marlborough fault system of New Zealand (Little and Roberts, 1997), the Whittier fault in the Los Angeles Basin (Wright, 1991), and the San Jacinto fault of southeast California (Armbruster et al., 1998). Our study of the Hosgri fault is the first quantita- tive restoration of digital subsurface structure-contour maps to lateral ter- mination of a regional strike-slip fault. Slip on a right-lateral fault can be accommodated by fault-parallel extension on one side of its termination and by fault-parallel contraction on Geology; November 1999; v. 27; no. 11; p. 1039–1042; 5 figures. 1039 Block rotation and termination of the Hosgri strike-slip fault, California, from three-dimensional map restoration Christopher C. Sorlien* Marc J. Kamerling Institute for Crustal Studies, University of California, Santa Barbara, California 93016, USA Drew Mayerson Minerals Management Service, 770 Paseo Camarillo, Camarillo, California 93010, USA ABSTRACT The Hosgri fault is located immediately offshore of south-central California and is part of the transform boundary between the Pacific and North American plates. This fault terminates to the southeast into east-trending folds and reverse-separation faults of the western Transverse Ranges. Our new structure-contour maps of deformed horizons show a spatial relationship be- tween faulting and folding consistent with right-lateral slip. Restoration of these digital maps quantifies post-Miocene right-lateral slip across the southern Hosgri fault to be 3.5 km. This slip is absorbed by folding, thrust overlap, and clockwise vertical-axis rotation of elongate blocks between strands of the fault. The restored part of a block located to the east has rotated 8° clockwise. Extrapolating this restored rotation to the 50 km block length produces an esti- mate of 7 km of dextral shear for a total, including the 3.5 km fault slip, of 10.5 km of post- Miocene displacement. Our three-dimensional approach precludes interpretations for reverse slip on the Hosgri fault, and is not consistent with models for more than 80 km of late Cenozoic right-lateral fault slip. *Mailing address: 61 Western Highway, Tappan, New York 10983. Email: chris@quake.crustal.ucsb.edu. 25 km Faults Transverse Ranges SMB 35°N 34°N 121°W 120°W 34°N 35°N 121°W 120°W CR San Andreas fault Hosgri fault SSF SBC Fig. 2 SYM California PSB AH SRM D V Platforms SB MB SZ SYM HF PSB Rotation of SYM and SMB E SYM SMB HF Rotation of Santa Ynez Mountains D HF SMB SYM No Rotation C A HF SMB SYM Past Position B Piercing Point Shortening (West Plunge) Clockwise Rotation Left Slip Right Slip Thrust Slip Figure 1. A: Location map of south-central California. AH— Amberjack high (northeast-trend- ing circles), CR—Coast Ranges; D—Diablo Canyon nuclear power plant, PSB—Point Sal block, SB— Shell Beach, MB—Morro Bay, SBC—Santa Barbara Channel, SMB—Santa Maria Basin, SRM— San Rafael Mountains, SSF—San Simeon fault, SYM—Santa Ynez Mountains, V—Vandenberg Air Force Base. Shading representing inferred right-lateral shear zone is labeled SZ. Offshore faults are mapped on middle and late Miocene horizons. B: Past posi- tion of blocks; not to scale. HF— Hosgri fault. C: Termination of right-lateral faults with no vertical- axis block rotation. D: Same as C except faults terminate into clock- wise-rotating block. Shortening decreases to east between fault intersections (folds plunge east). E: Nonrotating faults bounding clockwise-rotating slats terminat- ing into clockwise-rotating Santa Ynez Mountains block.